High density electronic systems such as the types developed for aerospace applications are normally formed of standard sized two sided electronic circuit modules typically having over 25 in.sup.2 of printed wiring board area on each side. These circuit modules require a combination of mechanical and thermal properties which enable the system to tolerate extensive thermal cycling and environmental stress without electronic failures. Differences in material properties among the various components in a module can lead to such failures during normal operation.
For example, integrated circuits are often packaged in leadless ceramic chip carriers formed of aluminum oxide (Al.sub.2 O.sub.3). Differences in the coefficient of thermal expansion (CTE) of a chip carrier and the printed wiring board to which it is attached ca lead to cracking of the intervening solder joints. Normally the printed wiring board is made of a low modulus or compliant material, e.g., an epoxy-glass fiber composite, which is bonded to a relatively stiff substrate such as a composite metallic structure. Thus the thermal expansion characteristics of the compliant printed wiring board are influenced significantly by the properties of the underlying substrate.
Problems of differential thermal expansion become magnified as the density of circuit components increases. With surface mount, i.e., leadless chip, carrier technology component and interconnect spacings are very small. For example, over 60 ceramic integrated circuit carriers may be mounted on each side of a six inch square substrate. As the amount of power dissipated per unit area increases a greater amount of heat is transmitted through the printed wiring board to the underlying substrate. Such high wattage circuit board modules may exceed 2.5 W/in.sup.2. Prior substrate designs have avoided component failures which might result from this excessive heat generation by limiting the power density and by attempting to match the CTE of the substrate material with that of the chip carrier over a large temperature range to avoid solder joint failure
Vibration is a common environmental problem which can cause failures in high density electronic systems. For example, high levels of alternating bending and shear stresses are induced on solder joints during module vibration. These stresses can result in premature failures of solder joints. Because the amplitudes of these vibration responses are a function of material stiffness one answer to this problem has been to increase stiffness by forming thicker substrates. For large modules, e.g., having 36 in.sup.2 of printed circuitry on each side, copper-Invar-copper substrates with thicknesses of 100 mils or more have been necessary to assure solder joint reliability in severe vibration environments. Another solution for mitigating the vibrational problems has been the introduction of structural stiffeners to increase the overall rigidity of the module. While attachment of such stiffeners may allow for a slimmer substrate, this approach increases fabrication time and cost. It also reduces the power dissipation level as well as available space for integrated circuits.
Conventional high power circuit board modules have successfully used bimetallic structures such as copper-Invar-copper or copper-molybdenum-copper systems to satisfy requirements for stiffness and thermal conductivity. The thickness of the relatively thin copper layer may be varied in order to match the CTE of the composite substrate with that of the chip carrier. The central Invar layer is formed of sufficient thickness to reduce vibrations which may cause solder joint fatigue. However, the high density of these metal based systems results in very heavy modules when, at the same time, overall system weight is a competing design constraint which limits the allowable thickness of the module.
Furthermore, since the Invar layer has a relatively low thermal conductivity each copper layer forms the primary path for dissipating heat generated by components on one side of the substrate. Thus in higher power systems a relatively thick metal layer is necessary to effectively dissipate the heat. This poses another competing design constraint, i.e., in applications where it is necessary to reduce the copper to Invar thickness ratio in order to more closely match the CTE of the substrate with that of the chip carriers.
While it might appear desirable to form the substrate from the same material as the chip carrier in order to better match the CTEs, most chip carriers are formed of aluminum oxide which is a brittle ceramic having marginal thermal conductivity. The brittle nature of the material make it highly susceptable to fracture during manufacture and handling.
Generally, it is the desire of the art to increase the density of electronic circuitry and to further minimize the overall size and weight of electronic systems. Problems of heat dissipation, differential thermal expansion and vibrational stress are limiting factors which have resulted in thick substrates formed of very dense materials. It would be advantageous to provide a relatively lightweight and high stiffness, high thermal conductivity substrate capable of accomodating a higher density of integrated circuits without further increasing the overall size and weight of the circuit module.